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November 30, 2017 -- CHICAGO - Applying iterative reconstruction to CT scans enabled the acquisition of coronary artery calcium (CAC) scores that were just as accurate as those from a standard protocol but with half the radiation dose, according to a study presented on Wednesday at RSNA 2017.

Researchers from Sapienza University of Rome led by Dr. Andrea Laghi examined the coronary CT angiography scans of approximately 100 patients at standard radiation dose with filtered back projection and at half-dose with iterative reconstruction. Both protocols led to a similar characterization of coronary artery calcification.

"Iterative reconstruction at 60% and 70% showed comparable results in terms of Agatston score and number of coronary plaques, with minimal differences in categories of risk classification," presenter Dr. Damiano Caruso told attendees at the meeting. "Patients may benefit in the future from half-radiation-dose coronary artery calcium score acquisition with iterative reconstruction."

Reducing radiation dose

Dr. Damiano Caruso from Sapienza University of Rome.

Coronary artery calcium scoring continues to grow in popularity as a noninvasive method for predicting coronary artery disease (CAD), especially for patients at intermediate and low-to-intermediate risk, Caruso said. One way to further increase the feasibility of the approach is to lower the CT radiation dose it requires.

"A challenge with reducing tube voltage is that it influences the calcium Hounsfield units (HU) and amplifies blooming artifacts, which often results in an overestimation of calcium scoring," he said.

Iterative reconstruction can help uphold the strong predictive value of calcium scoring at lower radiation doses by improving signal-to-noise ratio and image quality, he said.

In this prospective study, Caruso and colleagues obtained the calcium scores of 101 patients without known CAD on both half-dose and standard-dose coronary CT angiography exams (LightSpeed VCT, GE Healthcare). They reduced the radiation dose in the half-dose group by lowering the tube current to 85 mA from the standard 170 mA, without altering the tube voltage (120 kVp). The resulting effective dose was 0.64 mSv for the standard protocol and less than half of that for the half-dose protocol, at 0.31 mSv.

The researchers reconstructed the half-dose CT scans using iterative reconstruction (adaptive statistical iterative reconstruction, or ASIR; GE) from 10% to 100% power at 10% intervals. For image analysis, they used automated software (SmartScore 4.0, GE) to mark all pixels with a density greater than 130 HU and an area greater than 1 mm2 as calcifications. The software additionally recorded the Agatston score, mass, volume, and number of coronary plaques.

After excluding the 12 patients who did not undergo the half-dose CT exam, the team found no statistically significant differences between coronary artery calcification using the standard and half-dose protocols with iterative reconstruction at 60% and 70% strength.

Iterative reconstruction at 80% and 90% strength also produced similar mean Agatston scores to the standard protocol, although it failed to detect the same number of coronary plaques. Furthermore, the risk classification of the coronary plaques was analogous between the two protocols at both 60% and 70% strength.

There were a few limitations of the study, including a relatively small population size and the use of only one type of iterative reconstruction algorithm from a single vendor, according to Caruso.

Attendees further inquired as to the effect of patient size on this particular type of iterative reconstruction, considering that larger patients would need different parameters than smaller patients.

"What we are showing is the impact of iterative reconstruction itself -- that it improves detection [of coronary artery calcification] for each individual patient and not between patients of different sizes," Caruso said.

Currently, the data are still being refined as more patients join the study before recruitment closes in December this year. The team believes that further studies with different vendors and algorithms as well as image quality analysis would help validate the results.